(Alex Deakin is a BBC weather forecaster and a keen sports fan. His previous post for 23 Degrees discussed the weather at the British Open. Here Alex explores how the weather will affect the start of the Rugby world cup in New Zealand, which kicked off earlier today)

Although it snowed heavily only last month in New Zealand further dumps of the white stuff are not expected as Spring kicks in for the start of the Rugby World Cup

Image courtesy of wikicommons

Being surrounded by water New Zealand, like the UK, has a mild climate for its latitude. The Islands have some of the best skiing in the southern hemisphere so they get some snow but that's thanks to the huge mountains. Snow at low levels is rare. When it snowed in August in Christchurch it was described as a once in 30 year event, so it's about as rare as a New Zealand rugby world cup win!

Another cold snap hit NZ at the start of Spring (Meteorologically speaking that's the start of September) when temperatures again dropped to freezing but now temperatures have climbed and this weekend looks fairly typical with some sunshine on Saturday and a more showery picture on Sunday. Temperatures look pretty average for the start of the tournament too, peaking in the mid to high teens.

As the tournament gets underway rain and wind will be the major weather players. Like the UK New Zealand regularly gets flown over by areas of low pressure or depressions in Spring and Autumn, these bring spells of rain and strong winds.

A strong wind will have an impact of the kicking game (penalties, conversions etc). The stadia play a big part here too with some designed to shelter the playing surface (some of the grounds being used have a roof) but other, older ones likely to create their own swirling winds and interesting micro-climates.

Rugby players are well known to be a tough bunch and a bit of rain won't hurt, however a wet pitch and a slippery ball will have an impact. A wet game is usually a low scoring game, and with so many of the group games expected to be rather one sided this could be a bit of a leveller. i.e. a shock result is more likely if its raining.

So the weather will have an impact on the rugby world cup, but if it's a typical Spring in New Zealand we shouldn't expect any snow. For updates on the weather for the games try metservice, here you can catch my old colleague Dan Corbett (yes that's where he's gone) giving video forecasts.

On the other side of the planet, surfers are waiting. Fortunately, Hawaii isn't such a bad place to wait, being home to "the most beautiful beach in the world", luaus, very blue water, and more pineapples than you can shake a cocktail stick at. It is currently the season for big waves in Hawaii, but those keen to risk their lives on the biggest waves of them all like what was expected to kick start the Eddie Aikau competition in Waimea Bay Hawaii rely on the conditions being just right.

Why Hawaii? Is this just an excuse for an exotic holiday? You can surf in lots of places, but these isolated islands win the prize for some of the biggest waves on the planet because of two things. They're in the right place to intercept the huge swells crossing the Pacific Ocean, and they're the right shape not to waste any of the energy heading their way. I suppose that sunny warm islands are good for surfer street cred (can you imagine the Beach Boys in Alaska instead of California?), but it turns out that's just a convenient coincidence.

It all starts with a big storm in the north Pacific. Waves grow in storms because the water surface is pushed along by the wind. You can see ripples form if you blow sideways on the top of a cup of tea, and Ocean waves start in the same way.

If the wind keeps pushing on the waves, they keep growing. But storms make complicated choppy waves, terrible for surfing on. This is why it's so important that Hawaii is far out in the middle of a very deep ocean. As the newly-formed waves race away from the storm, a sorting process happens. Waves with a long wavelength (a long distance from one peak to the next) travel more quickly than waves with short wavelengths. They can also travel further before they give up their energy. Storm waves can travel thousands of miles, and after such a long journey, the nasty messy choppy water has turned into smooth long tidy swells.

Our cool surfer dudes (who are still waiting) are actually waxing their surfboards close to the top of a huge mountain. If they could see for miles underwater, they'd probably have vertigo. Most of the Pacific ocean is about 5 km deep and the seafloor is fairly flat. The Hawaiian islands are tall volcanoes sticking up out of this vast plain. And this is the other reason that Hawaii gets big waves. Shallow water weakens waves, but this isn't a problem in Hawaii. These ocean swells reach the shore with most of their energy intact. If the right combination of conditions occurs, the waves breaking on the north shore of O'ahu can be 20 metres high. When the surfers finally catch their giant wave, they're surfing on moving energy from a storm that happened two thousand miles away, and 1-2 days previously. It sounds amazing, but I think I'll stick to watching.

About this blog

Orbit: Earth's Extraordinary Journey explores the relationship between the Earth's orbit and the weather. Previously '23 Degrees' (working title); on this blog the weather community were invited to discuss their experiences of severe weather as and when events developed and share their iwitness footage throughout 2011. The audience were provided with an insight to the making of the series and exclusive behind the scenes footage. Follow us on Twitter.

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